(a) Field of the Invention
The present invention relates to a method and a device for automatically tightening bolts (or nuts) under optimum conditions.
(B) Description of the Prior Art
As for tightening of bolts, the torque method and the rotative angle method have heretofore been generally known.
The torque method, which is based on the assumption that the axial force on the bolt is proportional to the torque required to turn the bolt; is adapted to continuously detect the torque so as to control the tightening due to the axial force on the bolt.
However, the proportional relationship between the axial force on the bolt and the torque considerably changes owing to other factors than the bolt such as the characteristics of the tightening tool and variations in the friction coefficient between the bolt and the member to be clamped due to deposition of dirt or oil. Therefore, such axial force varies from bolt to bolt and it is difficult to obtain a predetermined axial force.
It is the above mentioned rotative angle method which has improved the torque method. Thus, it makes use of the fact that the amount of elongation of the bolt is proportional to the angle through which the bolt is turned. According to said second method, the bolt is turned through a fixed predetermined angle after bolt head comes in contact with the tightening bearing surface of a member to be clamped, in order to reduce variations in the axial force on the bolt proportional to the elongation of the bolt.
Even with this method, however, since it is very difficult to ascertain whether or not the bolt comes in accurate contact with the tightening bearing surface of the member to be clamped, the usual practice is to detect the time when about 1/3 of the tightening completion torque is reached (which is referred to as the snug point), followed by further turning of the bolt through a fixed angle to complete the tightening.
Further, the tightening tools generally used in carrying out said methods comprise a motor to turn a bolt at high speed to reduce the tightening operation time. With such high speed rotation of the motor, however, even if a stop signal indicating the completion of tightening is given, the inertia of the motor shaft prevents instantaneous stoppage of the operation, resulting in the disadvantage of the bolt being over-tightened. In this case, the idea might be conceived of giving a stop signal to the motor a little earlier in consideration of the inertia of the motor shaft. Even with such a measure taken, however, it is difficult to obtain the desirable tightening force, since such error-producing factors as variations in the frictional resistance of the tightening bearing surface and in the motor rpm are invloved.